A typical switching-type power converter circuit operates by storing and releasing energy in various discrete capacitive and inductive components during each cycle of operation, where the time interval for each cycle is determined by the switching frequency. An increase in switching frequency reduces the storage time interval and the level of energy stored in reactive components during any one particular cycle of operation. In principle this increase in frequency permits reduction of both the physical and electrical sizes of magnetic and capacitive storage elements for any particular power capacity.
Inasmuch as a significant increase in operating frequency of a converter promises a significant size reduction in the circuit components on the basis of energy storage per unit volume, the fact that the switching frequency of power converters has not increased dramatically is indicative of other constraints on the increase of operating frequencies. For example, the switching speed of bipolar semiconductor switching devices is limited by charge storage, thereby limiting the benefits to be achieved from high frequency operation. MOSFET switching devices may be used in place of bipolar devices; however, their switching speeds are limited by device capacitances and parasitic lead wire inductances.
Circuit components generally include parasitic electrical parameters that produce undesirable effects at high frequencies, and considerable design effort must be expended to compensate for them. For example, at high frequencies, the parasitic inductance and resistance of a capacitor alter its effect on the circuit. The inductor's interwinding capacitance, winding resistance, and core loss also limit the maximum practical switching frequency. Circuit board layouts also contribute numerous stray capacitances, inductances, and resistances which detract from power supply performance at high frequency. Because of these complicating factors, it is extremely difficult to produce a traditional pulse width modulated switching power supply circuit that operates at frequencies much above 500 KHz.
Despite the theoretical advantages of high frequency operation of power conversion circuits, these circuits have not been practical because of the many component and design problems related to operational difficulties at very high frequencies. One high frequency power supply which surmounts these difficulties is disclosed in U.S. Pat. No. 4,449,174 issued to N. G. Ziesse on May 15, 1984 and which is assigned to the same assignee as this application. That patent discloses a high frequency resonant power converter that can operate at high radio frequencies.
That circuit was designed to benefit from the advantages of high frequency operation by using the parasitic or adjunct reactive electrical characteristics of components as positive circuit elements. The term adjunct component is used herein to mean an electrical component characteristic inherent in a device, component, or length of conductor that is often considered a deleterious parasitic component but which is fully and positively utilized in the illustrative circuit herein embodying the principles of the invention. The switching device of the power train described in the Ziesse patent referenced above is driven by a separate or independent high frequency signal source. Voltage regulation is achieved by providing a range of frequency adjustment which is adjusted either directly or by feedback means to attain a desired output voltage level. Hence, the signal source driving the power switching device must be capable of operating over a sufficiently wide band of frequencies to provide the converter with a regulated output voltage over a range of output current and input voltage that depends upon the converter's usage.
The added circuitry of a separate high frequency driver stage to drive the power switching device and provide frequency adjustment for regulation adds complexity to the converter in terms of the component count. If the drive circuit has a wide bandwidth to accommodate the frequency adjustment range, it cannot be precisely matched into the gate, and much of the drive energy is wasted. To achieve the desired high efficiency, a drive circuit must have a narrow instantaneous bandwidth and be tunable over the frequency adjustment range. A separate, tunable drive circuit, however, adds still further to circuit complexity or component count.
A further consideration of a high frequency converter circuit is the performance of the rectifier circuit. A conventional rectifier design cannot perform satisfactorily at these high frequencies of operation. The rectified waveforms of a conventional rectifier operated at high frequency tend to have ringing transients in the waveforms due to the resonances caused by parasitic inductance and capacitance. These transients serve to lower rectification efficiency and are often difficult to filter from the output signal.
Another deficiency of prior art rectifiers when operated with sinusoidal voltage and current inputs is an input impedance characteristic that varies with frequency and load to the detriment of attaining a wide range of line and load regulation. In the case of a series resonant converter that regulates by varying switching frequency to vary a power path reactive impedance, the input impedance of the conventional rectifier changes in such a way as to counteract the frequency variations utilized to achieve regulation.